Common multimeters are classified as analog (pointer) or digital. Most analog meters do not include an AC current range, so they cannot directly measure leakage current. Digital multimeters usually provide an AC current range and can therefore measure leakage on supply circuits. A digital clamp meter is the most convenient tool for measuring leakage current.
Types of Leakage
(1) Line leakage: leakage current that occurs when a circuit has no load connected and reduced insulation causes a current path to earth.
(2) Equipment leakage: leakage current that flows when equipment is energized due to the insulation resistance between internal components such as coils and earth.
Insulation resistance measured with any multimeter is not a standardized measurement. Proper insulation resistance testing requires a dedicated megohmmeter. Readings from a multimeter are only indicative and can differ significantly from actual insulation resistance, because most multimeters apply only about 9 V DC from internal batteries, while AC supply voltages are typically 220 V or 380 V.
Measuring Leakage with a Digital Multimeter
If you are using a digital multimeter, set it to the AC current range.
(1) Line leakage measurement is relatively simple. Current measurements with a multimeter must be made in series with the circuit or device. With loads disconnected, open the conductor connection at the circuit breaker output and insert the meter in series between the breaker output and the conductor. Start on a higher current range if available; many modern digital meters are auto-ranging. The measured current is the circuit leakage.
Measuring equipment leakage is more complicated because enclosures are usually grounded. To measure leakage with a multimeter, isolate the equipment as required. With the equipment energized, connect one meter lead to earth and the other to the equipment chassis; the measured current is the leakage.
Detecting the Live Conductor with a Digital Multimeter
To detect the live conductor, set the digital multimeter to the AC ~750 V range. Wind one meter lead, either red or black, around your hand 3 to 5 turns, and touch the other probe to the conductor or outlet terminal.
This method relies on the distributed capacitance formed between the conductor and the human body. More turns increase that capacitance. Digital meters are generally more sensitive than analog meters, so they will show a larger reading.
The author frequently uses this method with four turns, obtaining about 57 V, which indicated the live conductor. With the wound lead held in place, touch the other probe to the neutral (N) and the PE protective earth in turn to verify.
If the digital multimeter displays 000, it indicates that the live, neutral, and PE protective earth have all been detected.
Capacitors pass AC and block DC. This detection method is intended to be safe and convenient and can be used in many situations.
